The present invention relates to doctor blades for direct contact with inking rollers provided with ceramic coatings. In particular, the doctor blade is useful for flexographic printing. The invention also provides for a doctor blade unit and a flexographic printer operating with the new doctor blade.
Although the invention is applicable to all doctor blades for direct contact with inking rollers provided with a ceramic coating the invention will be described mainly with reference to so called flexographic printing.
In the art of flexographic printing the amount of ink is volumetrically metered by the use of a so called anilox roller. This roller is usually constituted by a metal cylinder onto which a ceramic coating has been applied. The ceramic is normally applied by a thermal spray process. For the purpose of volumetric metering of the ink the ceramic surface is laser engraved in order to create uniform cells for carrying and transferring an even ink film onto a printing plate.
For the purpose of removing excess ink from the surface of the anilox or inking roller the state of the art techniques preferably use a so called xe2x80x9cchambered doctor bladexe2x80x9d. U.S. Pat. No. 5,735,210 is an example of prior art making use of such doctor blade concept. The patent describes a doctor blade unit for the inking system of a rotary printing press, said unit comprising a beam carrying two doctor blades in the approximate shape of a roof. These doctor blades define an ink chamber in co-operation with the inking roller, and the entrance blade, also called positive blade, having for a function of sealing the chamber, while the exit blade, also called negative blade, is the one wiping off the excess of ink.
The prior art doctor blades are made of different materials, such as PVC or other fibre-reinforced polymers, but are usually made of steel. Normally, hardened and tempered carbon steel or stainless steel is used, and such steel strip is relatively thin, typically 0.15 mm. Steel blades may have different designs and may either have rounded edges, maybe bevels, so called lamella blades, such as disclosed in for example U.S. Pat. No. 4,184,429. Since the hardness of the blade material is quite low compared to the hardness of the ceramic coating of the ink roller, about 600 micro-Vickers as compared to more than 1200 micro-Vickers, the blade wears off quickly in operation and must therefore be replaced at regular intervals. The quick blade wear causes an uneven wiping over time. Moreover, the productivity of the printing process is significantly reduced by such frequent blade changes.
Furthermore, depending on the type of ink and its content of pigment, abrasive wear of the blade can further decrease the lifetime, such as when using so called white ink containing TiO2 as a pigment which is very abrasive. There are-several reasons for changing the blades in printing operations, such as for example:
Excess of sliding wear, often related to high pressure load of the blade against the roll.
Edge wear of the blade on both sides of the chambered doctor blade leading to ink leakage.
Excess of abrasive wear when hard pigments are used in the printing operation.
In all these cases, as well as in combinations thereof, the sealing and wiping functions cannot be properly fulfilled. For these and other reasons there is a need for development of doctor blades with improved wear resistance, such as for use in flexographic printing.
The major object of the present invention is to provide a doctor blade for direct contact with an inking roller-provided with a ceramic coating, shell or sleeve.
Another object of the invention is to provide a doctor blade with extended lifetime thus improving operational productivity.
Yet another object of the invention is to provide a doctor blade with extended lifetime without changing the functionality of the anilox ceramic surface by any premature wear.
Still another object of the invention is to provide a doctor blade unit containing oppositely and toward each other directed doctor blades according to the invention.
A further object of the invention is to provide a flexographic printer including such doctor blade unit.
It has been found by experimental research that by the application of a ceramic onto a thin steel strip the wear resistance of the blade can be considerably improved without impairing the capacity of the inking roller to transfer the desired amount of ink. Thus, the improved wear resistance of the blade does not result in premature wear of the inking roller. Furthermore, the use of a ceramic coating on the blade eliminates any smearing effect which is typically encountered when steel blades without coating are used. Such smearing is caused by plastic flow of the steel contact surface against the ceramic inking roller. Moreover, the higher abrasive wear resistance of the ceramic as compared to steel greatly improves the lifetime of the blade when abrasive pigments are used.
Finally, the ceramic material which is less sensitive to load differences means that the typical edge wear encountered with steel blades will be significantly postponed. Accordingly, edge leakages are also significantly postponed.
It has been unexpectedly found that the ratio between the hardness of the ceramic coating on the blade and the hardness of the ceramic surface on the inking roller constitutes one of the key elements in the present invention. Thus, it has been found that the hardness ratio, hardness of blade ceramic versus hardness of ceramic roller, shall lie within the range from about 0.55 to about 0.8. Ratios lower than 0.55 do not result in significant advantages as compared to a steel blade which gives a typical ratio of 0.5 or lower. Ratios higher than 0.8 will start increasing wear on the ceramic inking roller. This could result in a decrease of the cell volume and thereby decreasing amount of transferred ink.
Another key aspect of the present invention is the choice of blade coating ceramic. The toughness of the ceramic was found to be another important parameter for the following reasons.
On a microscopic scale, the abrasive wear resistance of ceramic is directly related to the crack resistance or toughness of the material. Therefore, in order to resist against hard pigments a tough ceramic is to be chosen.
Furthermore, on a macroscopic scale, the blade must withstand machine vibration, chattering and contact loading at the edge of the ceramic layer without chipping.
Considering these aspects it has been found that materials within the Al2O3xe2x80x94ZrO2 family, with or without TiO2 addition, fulfill requirements of toughness and adequate hardness when applied by thermal spraying.
Accordingly, the invention provides for a doctor blade for direct contact with an inking roller provided with a ceramic sleeve or shell, said blade comprising a strip of metallic carrier material, wherein said strip, along one edge section thereof facing the inking roller, is provided with a ceramic coating having a wear-resistance lower than that of said sleeve and higher than that of said strip.
Said ceramic coating preferably has a hardness within the range about 55 to about 80% of that of said ceramic sleeve.
It is preferred that the strip forming the blade is constituted by a steel band having a thickness from about 0.05 to about 0.25 mm. The width of the band suitably varies from about 10 to about 60 mm.
The ceramic coating on the blade has preferably a thickness of between about 20 and about 100 xcexcm, typically about 50 xcexcm, and a width of from about 1 to about 8 mm, especially from about 4 to about 6 mm, such as about 5 mm.
In regard to the composition of the ceramic blade coating it is preferably based on Al2O3 and includes ZrO2 in an amount of from about 20 to about 50% by weight. Optionally, the ceramic coating can additionally contain TiO2 in an amount of up to about 20% by weight. These percentages are all based on the total weight of the composition.
According to a preferred embodiment the ceramic coating is provided with a tip bevel having an angle configuration conformed to the surface of the inking roller.
The invention also provides for a doctor blade unit for an inking system of a printing press provided with an inking roller having a ceramic coating or sleeve. This unit comprises a doctor blade holder carrying two doctor blades which are placed oppositely to each other and directed towards each other. These blades are provided with a ceramic coating as described above and are intended for direct contact with the inking roller. One blade is placed in trailing position and the other blade in butting position to the inking roller so as to define an inking chamber together with said inking roller. Trailing and butting positions correspond respectively to positive blade mode and negative blade mode.
Finally, the invention provides for a flexographic printer comprising an inking roller having a ceramic coating or sleeve, an inking system for this inking roller, a printing plate cylinder, and a pressure roller. In operation the printer accommodates a paper web travelling through a nip created between the inking cylinder and the pressure roller. In this flexographic printer the inking system comprises a doctor blade unit as described above, a printing ink container, an ink feeding conduit containing an ink feeding pump for transferring ink from the container to the inking system, and a return conduit for the return of excessive ink to the container.